The invention relates to a pressure control system for a cardiac assist device.
Congestive heart failure is one of the major causes of mortality and morbidity in the United States, affecting more than 2 million Americans. Pharmacologic therapy has prolonged survival and improved the quality of life for many patients. For cardiac patients who do not respond to conventional treatments, heart transplantation is an effective treatment. However, the shortage of donor hearts limits its application. Mechanical assistancexe2x80x94in the form of the intraaortic balloon pump (IABP)xe2x80x94has become commonplace for the treatment of acute heart failure. But to date, no forms of mechanical assistance for chronic heart failure (CHF) are commercially available.
During the last decade, left ventricular assist (LVA) systems have been used as a bridge to a heart transplant. These systems take over all the work of the heart and have been used for more than a year in many patients who have then gone on to be transplanted. The success of this prolonged cardiac support has led to ongoing clinical trials to evaluate the use of LVA systems as an alternative to medical treatment.
The system is designed for use in selected patients with advanced chronic congestive failure no longer responsive to pharmacologic management. Like the intraaortic balloon pump (IABP), the present invention is a left ventricular assist (LVA) system that provides diastolic augmentation to the failing left ventricle. However, the present invention differs from the IABP in a number of respects. The present invention is intended to remain in the body indefinitely, providing long-term cardiac support as an alternative to medical treatment. It is not a bridge to a heart transplant. Patients will be discharged from the hospital, live at home, and resume many normal activities.
The system consists of: the blood pump, an inflatable bladder sutured into the wall of the descending thoracic aorta, the percutaneous access device (PAD), a through-the-skin port that allows power and electrical signals to pass between the drive unit and the blood pump, and the external drive unit, which powers and controls the blood pump.
The blood pump has only one moving partxe2x80x94a diaphragmxe2x80x94and no valves. The pump has a stroke volume of up to 60 cc. To inflate the blood pump, pressurized room air is supplied from a wearable ( less than 5 lb.) battery-powered unit or a larger drive unit powered by household electricity. The air reaches the blood pump through an external drive line, attached to the drive unit, and an internal drive line implanted in the patient. The internal and external drive lines connect to each other through the percutaneous access device. The PAD also serves as a conduit for electrical signals that are transmitted from the heart to the drive unit through electrical leads.
Like the intraaortic balloon pump (IABP), a device whose effectiveness in providing circulatory support for days to months is well accepted, the present invention operates on the principle of diastolic augmentation. The system inflates the blood pump during diastole and deflates it just before systole. When deflated, the pump conforms to the inner wall of the aorta, functioning as a passive aortic graft.
In a patient with left ventricular failure, diastolic augmentation reduces left ventricular afterload and improves coronary perfusion. This leads to improved myocardial oxygen supply and demand balance. While the system takes advantage of the same operating principle and anatomic location as the IABP, the larger stroke volume of the present invention is expected to yield improved hemodynamic benefits. Unlike the IABP, which is designed for short-term, in-hospital treatment, the present invention is designed for long-term circulatory support of the CHF patient who will return home after recovering from implant surgery.
During the last decade, left ventricular assist (LVA) systems have been used as a bridge to a heart transplant. These systems are designed to take over all of the work of the left ventricle. In contrast, the present invention requires that patients have some functioning myocardium and can benefit from diastolic augmentation. Several features set the system apart from other LVA systems designed for long-term support:
It is an xe2x80x9con demandxe2x80x9d system. The present invention can provide either continuous or intermittent cardiac support, according to the physician""s determination of the patient""s needs. When the system is turned off, the patient does not need to be connected to the machine. Because other LVA systems are designed with valves, they must operate continuously to avoid the pooling of blood, which leads to thrombosis and emboli. The present invention can operate intermittently because it has no valves. Long-term tests of the present invention and its predecessors indicate that turning the assist device on after it has been off for some time does not result in emboli.
Patients will not be on anticoagulant therapy. The blood-contacting surface of the present invention is textured to encourage tissue ingrowth. Its biocompatibility has been confirmed in calf studies in which the blood pump was activated intermittently for periods up to 25 months. No surgical alteration of cardiac anatomy is necessary, so no functioning myocardium is removed when the present invention is implanted.
Following the success of LVA systems as a xe2x80x9cbridge to transplantxe2x80x9d in the past decade, trials are underway to evaluate these systems for long-term use as an alternative to medical treatment in CHF. A number of cardiomyopathy patients have recovered sufficiently to be removed from the transplant list and to have the LVA system removed. LVA systems are therefore also becoming known as a xe2x80x9cbridge to recovery.xe2x80x9d The present invention is not a direct competitor of these LVA systems. Rather, it is one of a family of mechanical support devices, each designed for particular patient needs. In the future, physicians will be able to choose from various models to provide the best match for each patient.
Electrode leads from electrodes are projected through the skin via the percutaneous access device (PAD) and the R wave of the electrocardiogram is monitored to control the fluid pressure during inflating and deflating cycles of the pump in synchronism with the natural heartbeat actions. By inflating the cardiac assist device during diastole and deflating the device during systole, the load on the left ventricle is reduced and the aortic pressure is raised to increase the blood flow to the coronary arteries. The cardiac motion needs to be sensed accurately to enable the device to be inflated and deflated correctly in accordance with the cardiac cycle. One way to sense cardiac motion is to measure the aortic pressure wave form and determine the occurrence of the dicrotic notch, which indicates when the aortic valve closes. It is desirable in the present invention to provide an apparatus and method for accurately sensing the blood pressure wave form within the aorta. It is desirable in the present invention to control the inflation and deflation timing of the blood pump or other cardiac assist device by periodically monitoring the aortic pressure while still providing partial cardiac assistance during the patient monitoring procedure to lessen the impact on the patient, and permit more frequent monitoring procedures to be performed. The monitored aortic pressures are stored, and the operational parameters of inflation and deflation timing of the blood pump for each subsequent heartbeat are adjusted in accordance with the stored aortic pressure.
According to the present invention, control means is provided for measuring arterial pressure of the patient during a monitoring procedure, sometimes referred to as a scheduled pressure measurement. The control means provides for adjusting the inflation and deflation timing of the pump for subsequent heartbeats in accordance with a program stored in memory of the control means based on the arterial pressure measured during the scheduled pressure measurement. Gas handling means is provided for inflating and deflating the pumping bladder in accordance with the evaluation of the arterial pressure measured by the control means.
The inflatable chamber of the cardiac assist device is disposed in a desired location with respect to the aorta of the patient. After connection of the inflatable chamber of the cardiac assist device to the drive means, a patient monitoring procedure is conducted to obtain a pressure measurement of the aortic pressure wave form. During the procedure, the inflatable chamber of the pump is first inflated to provide cardiac assistance during the monitoring procedure, and then partially deflated with the control means controlling the volume of fluid expelled from the inflatable chamber to provide a partially inflated chamber. The volume of gas expelled from the pump is calculated by monitoring the pressure drop across the deflation valve over an interval of time.
Accumulating the pressure drop with respect to time provides a value corresponding to the total volume of gas expelled from the pump. The total volume is monitored so that a predetermined volume is left remaining within the inflatable chamber. When the cardiac assist device is partially filled with gas, the deflation valve is closed to isolate the pump from the drive means while in a partially inflated condition. The pressure of the gas in the chamber reflects the aortic pressure of the patient when partially inflated and isolated from the drive means. This state is preferably maintained for at least a partial heartbeat and preferably at least one complete heartbeat. The pressure of the chamber is monitored continuously by a pressure sensor in the control means for the heartbeat cycle being monitored. A wave form of the aortic pressure of the heartbeat cycle is stored in memory of the control means. At the same time, an ECG signal is monitored and stored in memory of the control means. During the patient monitoring procedure, the control program stored in memory of the control means computes the systolic time interval, which is the elapsed time from the beginning of the QRS wave of the ECG signal to the closing of the aortic valve as indicated by the dicrotic notch of the aortic pressure. The ventricular assist control program uses the information provided during the patient monitoring procedure to adjust the inflation volume and timing for subsequent heartbeats. The patient monitoring procedure is repeated at scheduled time intervals and/or with changing heart rate conditions. The timed intervals and/or heart rate parameter conditions are fully programmable by the attending physician within preselected ranges and are stored in a patient parameter table located in memory of the control means.